A study of 200,000 Australians shows the habit cuts 10 years off the average smoker's life and is directly linked to two thirds of deaths in current smokers.

This is much higher than previous international estimates of 50 per cent.

The four-year analysis of health records in the Sax Institute's 45 and Up Study shows even moderate smoking is a major killer.

"We all know that smoking is bad for your health. But until now we haven't had direct large-scale evidence from Australia about just how bad it is," says study leader Professor Emily Banks, the scientific director of the 45 and Up study.

"We've been relying on evidence from other countries."

The study, supported by the National Heart Foundation in collaboration with Cancer Council NSW, shows risk increases with the number of cigarettes smoked a day.

The risk of death is doubled even among those smoking an average of 10 cigarettes a day, says study co-author Associate Professor Freddy Sitas from Cancer Council NSW.

The good news, says Prof Banks, is that stopping smoking at any age reduces the risk.

Smoking is the largest single preventable cause of death in Australia and kills 15,000 people a year, says the Heart Foundation's Dr Rob Grenfell, who worked on the study.

"People need to realise that smoking is a dangerous activity," he said.

"There's no safe level of smoking and there's no such thing as social smoking."